Method of assembling plastic and metal parts



March 3, 1970 J. KING' E A!- METHOD OF ASSEUBLING PLASTIC AND METAL PARTS Filed Aug. 24, 1967 I x I l INVENTORS JULI KING GEQ E .H OD. m

ATTORNEY United States Patent US. Cl. 29-629 2 Claims ABSTRACT OF THE DISCLOSURE A method of assembling metal and plastic parts in which one or more metallic elements are force-fitted into cavities provided within a molded thermoplastic housing. The elements are fitted in such a manner that one or more portions of the housing adjacent each cavity are undesirably stressed by insertion of the element. By selectively heating the elements independently of the housing, the stressed portions only of the housing are warmed and stress relieved. The method may include the additional step of treating the stressed portions of the housing with a fluid having a swelling effect thereon, prior to the heating of the metallic element.

The present invention relates to a method of uniting a metallic element, such as an electrical contact to a thermoplastic housing, and more particularly, to retaining electrical contacts having projections thereon, within a housing which has been provided with cavities into which the contacts are force-fitted.

Heretofore various methods have been proposed for forming composite structures, such as electrical or electronic connectors, by supporting a metallic contact element within a molded thermoplastic housing.

In one well known prior art method, the metallic contact is merely force-fitted into an aperture provided in a molded thermoplastic housing and retained therein by friction between contact and housing. It has been found in practice that in connectors formed in this manner, the contacts often may be later pulled from the housing inadvertently and with little effort, thereby rendering the connector unusable. Efforts to increase the force with which the contacts are retained, have been directed generally toward increasing the interference with the housing. This approach has been found to be likely to unduly stress the housing, thereby causing undesirable distortion and often resulting in stress failure of the plastic during subsequent handling.

Attempts have been made to overcome the disadvantages referred to above by heating assembled connectors within an oven or liquid bath so as to both relieve the stresses caused by force fitting of the contacts and to promote flow of the plastic about various projections on the contacts. It has been found, however, that heating of the entire connector tends to deform and distort the housing to unacceptable limits. For connectors requiring extreme high precision involving little or no dimensional variations, this method of stress relief may be completely unusable. Distortion of the housing due to heating also tends to misalign the previously positioned contacts, thereby further reducing the acceptability of the treated prodnet.

The present invention seeks to overcome the disadvantages of the prior art methods mentioned above while providing a resultant connector structure wherein the pull out force which resists separation of the contact from the housing is multiplied several fold.

A further object is to provide an assembly of the type described in which flow of the housing plastic may be ice limited to desired areas immediately adjacent each inserted metallic element.

A feature of this invention is the direct heating of the metallic elements only, so that heating of the plastic is accomplished only be heat transfer from the elements.

A further feature is the use of induction heating apparatus to heat the metallic elements.

These and other objects, features, and advantages of this invention will be made more apparent by the following specification, when taken in conjunction with the accompanying drawing, wherein:

FIGURE 1 is a vertical sectional view taken through a housing block of an electrical contact inserted in place, and ready for treatment in accordance with this invention.

FIGURE 2 is a perspective view of a connector having portions broken away to show the contact in detail;

FIGURE 3 is the fragmentary enlarged view representing the manner in which a housing is deformed upon force fitting a contact therein; and

FIGURE 4 is a view similar to FIGURE 3, but representing an assembly formed in accordance with this invention.

The method according to the present invention is particularly adapted for use in the fabrication of electrical or electronic connectors of the general type disclosed for instance in US. Patent 3,214,723 and designated as 1 in FIGURES 1 and 2. While the structural design of connector 1 is not an element of the present invention, it is briefly discussed below in order to facilitate understanding of the application of the invention thereto.

Connector 1 is shown in FIGURES 1 and 2 as comprising generally a molded thermoplastic housing block 2 and spaced electrical contact elements 3, only one of which is shown.

Housing block 2 includes a base portion 4, which is provided with a plurality of spaced apertures 5, each of which is adapted to receive one of contact elements 3. Apertures 5 open into cavities 6, which are defined by integrally formed parallel walls 7 and 8 and separators 9. Separators 9 are shown as being provided with aligned slots 10, which together define a channel adapted to slidably receive a printed circuit board or the like, not shown, having a plurality of spaced electrical terminal elements thereon. Housing block 2 may be formed readily by molding any suitable thermoplastic dielectric material.

Contacts 3 are shown in FIGURES 1 and 2 as including a terminal portion 11, which is adapted to be forcefitted within aperture 5, and a spring contact portion 12, which is adapted to be positioned within cavity 6 to engage an inserted circuit board. Contact portion 12 is formed to permit resilient flexure theretof upon insertion of a printed circuit board into slots 10. It will be understood that when a printed circuit board is in inserted position, the contact portion 12 of each of the contacts is disposed in electrical contact with an appropriate terminal element on the circuit board. Terminal portion 11 is shown in the FIGURES 1 through 4 as being provided with dimple-like protuberances 13 and corresponding recesses 14 for the purpose of locking the contact within aperture 5. Protuberances or detents 13 and recesses 14 may be formed by punching or embossing the contact either during or after formation thereof. For the purposes of the present invention, any desired number of projections of any desired configuration may be provided, and the contact may be formed of any suitable resilient, electrical1 -conductive material.

In the practice of the method of the present invention,

the first step includes force-fitting one or more contacts 3 into preformed apertures 5 provided in a molded thermoplastic housing block 2 to position the contacts as indicated in FIGURE 1. FIGURE 3 illustrates the manner in which side wall 15 of aperture 5 is deformed or furrowed by projections 13 when the contact terminal portion 11 is moved in the direction of arrow 16 into the seated position shown. FIGURE 3 indicates that aperture wall portions 17, which are in direct surface engagement with protuberances 13, are subject to a particular degree of stress concentration. The degree of stress imparted to aperture wall portions 18 is less than that imparted to wall portions 17 in that the former are not physically prevented from returning to their original positions after passage of projections 13. Also, it will be understood that aperture wall 19 is subject to a relatively uniformly distributed compressive stress due to contact with the recesscontaining surface of terminal portion 11.

Thus, it may be seen that insertion of a contact permanently deforms part of the housing block in the areas of highest stress concentration, i.e., wall portions 17, creating a frictional engagement with protuberances 13 to lock the contact within the housing block. In any given instance, the force necessary to pull out a contact seated in this manner depends upon the relative dimensions of the contact and aperture and to a certain extent, upon the creep characteristics of the plastic material from which the housing block is formed.

This invention makes it possible to greatly increase the pull-out force of contacts which have been forcefitted into a housing block in the manner described above, without encountering. undesirable distortion or weakening of the housing block.

According to the method of the present invention, the stressed portions of the housing block into which contacts have been force-fitted are indirectly warmed by rapidly heating the contacts independently of the housing. Heating of the contacts may be effected, as generally indicated in FIGURE 1, by an induction heating apparatus, including an alternating current source 20 and coil 21, which is adapted to be disposed about a housing block base portion 4. Direct heating of the contacts may also be effected in various other ways, as for instance by electrical resistance heating or by coupling the terminal portions of the contacts to a controlled heat source.

The effect of heating the contacts as described, is generally illustrated in phantom line in FIGURE 3. Since the dielectric material does not respond to the current flow in the coil 21, heat is transferred to the housing block principally by conduction in those areas adjacent the contacts. Thus, flow and relaxation of the housing plastic caused by applied heat is substantially limited to those areas of high stress adjacent each contact. The application of heat to the housing in this manner also permits the plastic housing material to flow into contact recesses 14 thereby further anchoring the contact. Further, it appears that heat transferred to aperture wall portions 18 permits the plastic memory of those portions to at least partially erase the furrow created by insertion of the contact. Thus, heating of the contacts serves to simultaneously reduce the stress induced in the housing block during contact insertion and to increase the bulk of plastic opposed to contact removal, thereby increasing the removal pull-out force. It can be understood now that by employing induction heating, only a small portion of the plastic forming the housing block is involved, thereby distortion and change in the dimensions and configuration of the connector are avoided.

In any particular case, regulation of the induction heating apparatus will be in accord with the type of material employed in the housing block and with the stress levels developed within the block. Preferably, however, the temperature to which the contact is heated should be suflicient to permit plastic flow of the stressed portions of the housing without effecting melting or charring of the palstic. Heating should continue until the existing stress levels of the plastic are decreased to a point at which stress can no longer be detected by solvent stress measurement techniques.

I In practicing the steps of the present invention, I take advantage of two phenomena, i.e., that heating of a previously molded thermoplastic material, as for instance nylon, polycarbonate or polypropylene, reduces the intrinsic viscosity thereof by breaking some polymer chains, and that low viscosity thermoplastics of this type are inherently more subject to attack by a solvent or a swelling agent. Thus, as one step of the method according to the present invention, a fluid having a solvent or swelling ac tion on the plastic may be applied to the stressed portions of the housing previously relieved. The solvent or swelling fluid may be either a liquid or a gas and may be applied, if desired by dipping the connector in a liquid bath for a period of time suflicient to permit the liquid to wet the walls of the contact receiving apertures by capillary action. The external surfaces of the connector may then be washed to remove the treating fluid to prevent injury to the housing.

As an additional step, the treated connector may be selectively heated by induction heating of the contacts. The effect on the plastic due to solvent treating and subsequent heating is illustrated in FIGURE 4, wherein the plastic is shown to flow into intimate contact with the terminal portion of the contact to efiectively hold the contact within the housing block.

By again employing inducion heating subsequent to solvent or swelling agent-treatment, only a relatively small portion of the plastic is heated, and dimensional changes are avoided again. Also, it will be apparent that by employing induction heating in the step described above, the action of the solvent or swelling agent may be restricted to the plastic immediately adjacent to the contacts, whereby degrading of the housing block as a whole is prevented.

It is presumed that the presence of the solvent or swelling agent tends to plasticize the previously stress relieved portions of the plastic material, allowing them to swell and more readily form around the terminal portion of the contact. Subsequent heating apparently reduces any stress imparted to the housing block due to swelling, and facilitates additional plastic flow. It has also been hypothesized that heating of the contacts acts to drive the swelling agent into the plastic, so as to increase the swelling action thereon and to form a thin crustlike surface or layer on the largely amorphous plastic substrate as well. This crust would be generally in direct contact with the terminal portions of the contacts. The presence of such a thin crust would permit wider distribution of shear stresses to the plastic forming the housing block, and would appear to explain the significant increase in force necessary to remove contacts from a conenctor treated in this manner.

The method of the present invention may be fully understood by referring to the following examples:

These examples employed a connector formed by forcefitting 2.00 contacts into preformed apertures provided in a molded housing block formed from a polycarbonate sold by the General Electric Company under the trade name Lexan. The minimum initial intrinsic viscosity of the plastic was about 0.52, and portions of the housing block adjacent to contacts Were determined to be under compressive stress of between 1,750 and 2,000 p.s.i. The contacts were formed from phosphor bronze plated with gold. It was determined that an average of about 17 pounds of force was necessary to pull out a contact.

EXAMPLE 1 A connector formed in the manner set forth above, was selectively heated by induction heating of the contact elements until zero stress levels were determined by solvent stress-measurement techniques. It was found that there had been a measureable flow of plastic about the protuberances and into the recesses of the terminal portions of the contacts, and that there had been no dimensional changes in the housing block. Further, the intrinsic viscosity of the previously stressed portions of the plastic had been reduced to about 0.51. After the contacts had been permitted to cool, attempts to pull out the contacts indicated that the average force necessary was on the order of about 27 pounds.

EXAMPLE 2 A connector which had been stress relieved, as set forth in Example 1, was dipped in a bath of alcohol for a sufficient period of time to permit the walls of the contact receiving apertures to be wetted by capillary action. The connector was then removed from the bath and cleaned. The contacts were again heated and permitted to cool. Inspection indicated that the plastic had completely enclosed the terminal portions of the contacts and it was found that on the average a force in excess of 50* pounds was required to remove each contact.

The invention has thus been described but it is desired to be understood that it is not confined to the particular forms or usages shown and described, the same being meely illustrative, and that the invention may be carried out in other ways without departing from the spirit of the invention; therefore, the right is broadly claimed to employ all equivalent instrumentalities coming within the scope of the appendant claims, and by means of which objects of this invention are attained and new results accomplished, as it is obvious that the particular embodiments herein shown and described are only some of the many that can be employed to obtain these objects and accomplish these results.

What is claimed is:

1. A method of assembling metal elements to plastic parts, in which at least one metal element is retained within a cavity in a housing block of molded themoplastic material, including the steps of forcing said metal element into the said cavity within said housing block in an interference fit so that at least one portion of said housing block is mechanically stressed by engagement with said metal element; heating said metal element and said one portion of said housing block to a temperature sufficiently higher than the unstressed portions of said housing block to relieve the mechanical stress in said one portion.

2. The method of claim 7, wherein heating of said metal element is accomplished by electrical induction means and the portions of said housing block adjacent to said metal element are heated by virtue of proximity to said metal element.

FOREIGN PATENTS 12/1963 England. 2/ 1949 England.

JOHN F. CAMPBELL, Primary Examiner R. W. CHURCH, Assistant Examiner US. Cl. X.R. 

